1. Field of the Invention
The present invention relates to a module battery of a sealed alkaline storage battery having a comparatively large capacity.
2. Description of the Prior Art
Sealed alkaline storage batteries, which typically include nickel-cadmium storage batteries and nickel-metal hydride storage batteries, are widely used as power supply sources for portable equipments such as video cassette recorders, lap-top type computers and portable telephones owing to their high energy density and reliability. These batteries have metal casings of cylindrical or rectangular shape, a capacity of about 0.5 Ah to 3 Ah, and are configured of cells to make up a small-sized sealed alkaline storage battery. In practical applications, several to several tens of cells are generally contained in a synthetic resin casing or tube.
These small-sized sealed alkaline storage batteries have a battery capacity as small as about 0.5 Ah to 3 Ah, and therefore generate only a small amount of heat per cell at the time of charging or discharging. As a result, in the case where they are used in a resin casing or tube, an appropriate balance or equilibrium is maintained between heat generation and heat discharge. Therefore no significant problem has been posed with regard to a temperature increase of the battery. Also, although the electrodes of the alkaline storage battery expand as a result of repetitive charging and discharging, no serious problem such as casing deformation due to electrode expansion has been posed in view of the fact that the casing is made of a metal having a durability.
Nevertheless, there is an ever-increasing demand for medium and large-sized batteries (the medium-sized battery is defined as those from 10 Ah to 100 Ah in capacity, and the large-sized one as those 100 Ah or more in capacity. The number of cells used is several to several hundreds for either type) which is high in energy density and in reliability as a mobile power supply for various devices including home-use appliances and electric vehicles. The open-type nickel-cadmium storage battery and the lead-acid storage battery are used for energy storage and as an uninterruptive power supply. In spite of these advantages, these batteries have the disadvantage of the need of troublesome maintenance such as for filling an electrolyte solution during the lifetime. In view of this, there has been a demand for a battery suitable for a mobile power source for various applications including home-use appliances and electric vehicles, which is free from maintenance, by having a sealed configuration.
As explained above, in the case where an alkaline storage battery is used as a mobile power supply for various devices including home-use appliances and electric vehicles, the battery is required to be sealed and increased in size to medium or large scale at the same time. More specifically, it is necessary to increase the electric capacity of the cell and to connect a multiplicity of the cells in series in order to increase the electric capacity and voltage of a unit, while sealing them.
A battery generates Joule's heat and reaction heat due to electrode reaction at charging and discharging operations. A module battery including a series of cells having such a large capacity or a pack battery including a series of the module batteries is configured of several tens to several hundreds of the cells arranged contiguously to each other. The cells, with an increased electric capacity and sealed configuration, increase in the amount of heat accumulation, with the result that heat dissipation out of the battery is retarded and the generated heat is accumulated within the battery. Consequently, the internal temperature of such a battery rises by a degree more than that of a smaller battery.
In order to obviate the above-mentioned problems, a heat dissipation device for the storage battery system has been suggested by Japanese Laid-Open Patent Publication No. Hei 3-291867 configured of a multiplicity of cells generating heat at the time of charging, in which each cell includes positive electrodes, negative electrodes and an electrolyte, and in which a space for allowing air flow is formed between the cells with a ratio of space width to cell width set to a range of 0.1 to 1.0.
However, the conventional sealed alkaline storage battery for a mobile has the following problems:
(1) In a lead acid storage battery, the internal pressure is increased through a charging operation by approximately 0.05 MPa even when the battery is of a sealed type, while the increase of the internal pressure caused in a charging operation of a sealed alkaline storage battery is 0.2 to 0.4 MPa. When the battery is used as a mobile power supply in the high temperature environment in the open air for a long period, in particular when it is used or left in a charged state, the battery casing is exposed to continuous application of the internal pressure of approximately 0.2 to 0.4 MPa, and a battery casing with low rigidity and small strength has a possibility of damage due to creep deformation. Furthermore, in the case where 1,000 or more cycles of the charging and discharging operations are conducted in the high temperature environment in the open air, the battery casing has a possibility of being damaped owing to dynamic fatigue caused by variation in the internal pressure, and therefore, in view of long term reliability and safety, a module battery structure is desired in which a battery casing is prevented from being damaged due to the deformation.
(2) In the case where a battery casing is expanded and deformed due to the increase of the internal pressure, there occurs a space between a power generation element group and the battery casing. When the space exists between the power generation element group and the battery casting, the transferring rate of heat generated in the power generation element group to the battery casing is extremely decreased. Therefore, a module battery structure is desired in which a power generation element group is always made to be in contact with a battery casing by suppressing the deformation of the battery casing.
(3) For the application to a mobile power supply, a module battery including accumulation of approximately 5 to 40 cells, or two or more module batteries, that is, accumulation of approximately 10 to 300 cells, is used, and hence, it is necessary to provide means for decreasing the variation in the battery performances such as battery capacity among these several to several hundreds cells, improving the battery performances such as energy density and improving the mechanical strength in order to suppress displacement due to vibrations and to make constant the dimensional change caused during the charging/discharging cycles. Furthermore, since a plate and a binding member for binding the cells are used in the open air for a long period, it is necessary to prevent degradation due to corrosion.